Microporous membranes made of polymeric materials are used in various applications, for example, filter membranes and separation membranes for medical and industrial use, and separators, such as battery separators and condenser separators.
In particular, with the recent growing demand for secondary batteries as power supplies for mobile phones, mobile personal computers, and automobiles, there is also a growing demand for battery separators. However, battery separators made of conventional polymeric materials have insufficient characteristics, particularly unsatisfactory heat resistance and thermal shrinkage ratio.
Use of a blend of polyethylene (PE) and polypropylene (PP) or a higher-molecular-weight polyethylene (PE) to improve the properties of battery separators has resulted in still insufficient characteristics and particularly cannot achieve high heat resistance and low thermal shrinkage ratio.
Patent Literature 1 describes a high-molecular-weight PP defined only by an MFR≦1.2 g/10 min. However, the resulting membrane has unsatisfactory thermal shrinkage ratio.
Patent Literature 2 describes a separator made of polyolefins having different melting points. However, as shown in an example, a separator made of a PP having an MFR of 3 g/10 min and a high-density polyethylene (HDPE) having an MFR of 5.5 g/10 min does not have desired properties as a separator (for example, the upper limit temperature at which pores remain closed and thermal shrinkage ratio).
Patent Literature 3 describes a microporous membrane (separator) made of polyolefins having different viscosity-average molecular weights. However, a high-molecular-weight PE only is used, and there is no description of the properties of the separator.
Patent Literature 4 describes a microporous membrane (separator) made of a polyolefin, particularly a metallocene PE, having a residual Cl amount of 5 ppm or less and a viscosity-average molecular weight of 1,000,000 or more. However, the examples describe only a metallocene PE, and desired properties (for example, 150° C. puncture strength) cannot be achieved. There is no description of thermal shrinkage ratio.
Patent Literature 5 describes a microporous membrane (separator) made of a PE having a viscosity-average molecular weight (Mv) of 300,000<Mv<600,000, a PE having a Mv of 600,000≦Mv≦10,000,000, and a PP (150,000≦Mv≦700,000). However, the microporous membrane (separator) does not have desired properties as a separator (for example, thermal membrane-fracture temperature). There is no description of thermal shrinkage ratio.
Patent Literature 6 describes a microporous membrane (separator) made of a PE and a PP having a weight-average molecular weight (Mw) of 500,000 or more. However, the microporous membrane (separator) cannot achieve desired properties (for example, thermal shrinkage ratio).
Patent Literature 7 describes a microporous membrane (separator) made of two polyolefins each having a weight-average molecular weight (Mw) of 500,000 or more. However, the examples describe only HDPE, and desired heat resistance cannot be achieved. Furthermore, the resulting membrane has unsatisfactory thermal shrinkage ratio.
As described above, application of high-molecular-weight polypropylenes to improve, for example, the heat resistance of microporous membranes has been conducted but had insufficient effects.